In the field of semiconductor devices, there has been an increasing interest in the development of more intelligence based within the device, often referred to as ‘smart’ devices. The terminology used for ‘smart’ devices encompasses the association of analogue and digital circuitry with precise diagnosis. It is also generally desired to implement more intelligent features in the provision of smart high-power devices, in order to improve reliability and longevity of the device, which is known as problematic due to the increased stresses applicable with high power operation. One such smart high-power device is a lamp driver. In the context of the present invention, the term ‘lamp driver’ encompasses a driver circuit for filament lamps.
All known lamp driver integrated circuits (ICs), such as an MC33892 switch from Freescale™, etc. require the ability to support a high current upon switch ‘ON’ of the lamp. In this regard, and referring first to FIG. 1, a known process of a bulb heating up and cooling down is illustrated graphically 100. The graph 100 illustrates how a bulb current (in Amps (A)) 105 varies 115 versus time (in msec) 110. The bulb is initially illustrated as being turned ‘ON’, where the ‘turn-On’ current reaches a peak current of approximately 17 A. The bulb is left in an ‘ON’ state for approximately 100 msec's 120, during which time the current requirements drop to a dc current value of around 2 A, and then the bulb is turned ‘OFF’ 130. Notably, if the bulb is then turned ‘ON’ again 125, after say an ‘OFF’ period of 300 msec's, the bulb only draws 4 A.
However, the inventors of the present invention have recognised that even though the ‘bulb’ current drops from, say 17 A to 2 A in around 50 msec., a standard lamp driver requires a high current of (maximum) 45 A upon turn ‘ON’, which is maintained for say a maximum period of 80 msec. when it is stepped down to, say 5 A. This lamp driver current requirement 215 is illustrated graphically 200 in FIG. 2.
Similarly, if the bulb is turned ‘OFF’, the current limitation is reset and will be kept at a high level of 45 A again until the next turn ‘ON’ operation. Such high currents are very undesirable and significantly shorten the average life span of the lamp driver device.
It is known that some applications may employ pulse width modulation (PWM), where the cyclical current requirements may be set through a serial port interface (SPI). Employing a PWM mode of operation facilitates a significant reduction in the average current requirements of a lamp driver circuit. Here, PWM may be employed at a rate, say, of typically 200 Hz, and applied after the initial 45 A inrush current.
However, in implementing a PWM scheme, a digital circuit is required and configured to control the lamp driver in a real time manner. In this regard, the digital circuit provides control signals to the lamp driver, say 80 msec after the start of PWM period. Alternatively, the lamp driver needs to be configured to perform the PWM operation, which adds to the complexity.
Notably, such circuits cannot be employed with low PWM rates, such as a PWM at around ‘1’ Hz that would be suitable for flasher application or for reliability testing with cyclic short circuits, again at around 1 Hz.
The inventors have recognised and appreciated a further problem with lamp driver ICs, in that they are prone to cyclical short circuits, for example a permanent or erratic short circuit with repetitive turn-‘ON’. In this regard, the lamp driver circuit has no ‘memory’ of a previous PWM cycle, i.e. the current limit is reset at every turn ‘OFF’. Hence, known lamp driver circuits assume that the bulb is always cold (i.e. the motor has stopped or an inductance has been charged), and consequently they draw 45 A as a prerequisite upon switch ‘ON’.
In known lamp driver applications, it is also known that the current limit of a lamp driver power stage comprises two levels, one for the peak current and one for the dc level. Furthermore, this current limit is set to support the worst case current loads required by the lamp. Also, the current limit imposed on the driver current needs to be able to support an inrush current at each turn ‘ON’ of the lamp.
Furthermore, in a case of a ‘true’ short circuit, the device will potentially drive a high amount of current into the lamp at each turn ‘ON’. This situation creates high levels of stress in the IC package, thereby reducing the lifetime of the device.
Thus, there exists a need for improved protection during an ‘ON’ phase of the current driver, such as one suitable as a lamp driver and bulb arrangement, and method of operation therefor.